1. Field of the Invention
The present invention relates to a current biasing circuit. More particularly, the present invention relates to a current biasing circuit based on a current mirror architecture.
2. Description of the Related Art
An integrated circuit (IC) comprises many component blocks. Such blocks may need controlled current sources to supply stable and constant currents. For example, an operational amplifier may need a constant current of 1 mA.
FIG. 1 is a schematic diagram showing a conventional current biasing circuit 100. The circuit 100 is based on a current mirror architecture and supplys constant currents to component blocks of an IC. The current I11 is equal to the current I12. The ratio among the currents I12, I13 and I14 is decided by the aspect ratios of the metal oxide semiconductor field effect transistors (MOSFET) M14, M15 and M16.
The variance of the current I12 is proportional to the square power of the resistance variance of the resistor Rs because of the current-voltage characteristics of the MOSFET M12 operating in the saturation region. Therefore the reference current I12 is very sensitive to the resistance variation of the resistor Rs. Results of a computer simulation show that there is a 70% current variance when the temperature varies from −25° C. to 120° C. The reference current I12 may be driven out of specifications due to such a current drift, thus complicating circuit design.
FIG. 2 is a schematic diagram showing another conventional current biasing circuit 200. The bipolar junction transistor (BJT) Q2 counteracts the temperature-dependent resistance variation of the resistor RN2. The currents I21 is equal to the current I22, which means the gate-to-source voltage of the MOSFET M21 is equal to the gate-to-source voltage of the MOSFET M22. The gate terminals of the MOSFETs M21 and M22 are connected together. The BJT Q2 and the resistor RN2 are connected to the common voltage source VSS. Therefore the voltage across the diode-connected BJT Q2 (the base-to-emitter voltage of Q2) is equal to the voltage across the resistor RN2. The base-to-emitter voltage of the BJT Q2 is constant. The reference current I22 is equal to the constant base-to-emitter voltage divided by the resistance of the resistor RN2. Since both the base-to-emitter voltage of Q2 and the resistance of the resistor RN2 have negative temperature coefficients, the current drift of the circuit 200 caused by temperature variance is much slighter than that of the circuit 100.
However, the temperature coefficient of the base-to-emitter voltage of Q2 is more negative than that of the resistance of the resistor RN2. The base-to-emitter voltage of the BJT Q2 drops faster than the resistance of the resistor RN2 when the temperature rises. The current drift is still severe when there is a wide variance in temperature. Results of a computer simulation show that there is a 22% current variance when the temperature varies from −25° C. to 120° C.